Device for testing the tune of musical instruments

ABSTRACT

A device for testing the tune of musical instruments comprises an adjustable, active, band-pass filter for transmitting in-tune electrical signals and for rejecting out-of-tune electrical signals corresponding to tones of improper pitch produced by a musical instrument to be tuned. The device also includes an indicator operated in response to transmitted signals. The musical instrument, itself, may be provided with means for converting a generated musical tone to an electrical signal, which is then fed into the device, or the device may include such means.

United States Patent 1191 Iannone 1 1 DEVICE FOR TESTING THE TUNE OFMUSICAL INSTRUMENTS [76] Inventor: Gary L. Iannone, 1072 28th St.,

Ogden. Utah 84401 [22] Filed: Oct. 17, 1973 211 Appl. No.2 407,190

52 us. c1 84/454; 84/].16 51 1111.0 ..G10g 7/02 [58] Field of Search....84/454, 1.01, 1.16, DIG. 18,

[56] References Cited UNITED STATES PATENTS 1,690,279 11/1928 Craft84/464 3,204,513 9/1965 Balamuth 84/454 3,474.774 10/1969 Johnson ct a184/464 OTHER PUBLICATIONS C. M. Harris, Handbook of Noise Control,

McGrawHill Book Co., 1957, pages 16-26 and 1627.

1451 July 29, 1975 Handbook of Operational Amplifier Active RC Networks,Burr-Brown Research Corp., 1966, pages 72 and 73.

Primary Examiner.loseph W. Hartary Assistant Examiner-U. WeldonAtlorney, Agent. or Firn1Mallinckrodt & Mallinckrodt [57] ABSTRACT Adevice for testing the tune of musical instruments comprises anadjustable, active, band-pass filter for transmitting in-tune electricalsignals and for rejecting out-of-tune electrical signals correspondingto tones of improper pitch produced by a musical instrument to be tuned.The device also includes an indicator operated in response totransmitted signals. The musical instrument, itself, may be providedwith means for converting a generated musical tone to an electricalsignal, which is then fed into the device, or the device may includesuch means.

5 Claims, 2 Drawing Figures PATENTED JUL 2 9 I975 SHEET INSTRUMENTSBACKGROUND OF THE INVENTION Field The invention is in the field ofdevices useful for testing the tune of musical instruments.

State of the Art There have been numerous devices developed for aidingpersons in properly tuning musical instruments. Most of these devicesinvolve a complicated system for generating an electrical signal offrequency corresponding to the desired pitch of a musical tone and forcomparing the generated signal with a corresponding tone produced by themusical instrument concerned. The musical instrument is adjusted untilthe pitch of the tone concerned corresponds exactly with the generatedsignal, at which time it is in tune. In a particular instance (U.S. Pat.No. 2,958,250), a passive band-pass filter is used in connection withsuch a frequency comparison system to cut out the fundamental tone whena harmonic thereof is being compared.

SUMMARY OF THE INVENTION In accordance with the invention, a device fortesting the tune of a musical instrument utilizes an adjustable, active,band-pass filter to transmit electrical signals of preset frequency andto block other signals. Means are provided to indicate when a signal istransmitted by the filter. Such means may be, for example, a meter whichreads a particular value or reaches a maximum deflection, or may be aloudspeaker which produces a tone when a signal is transmitted.

The device also includes means for feeding, to the band-pass filter,electrical signals corresponding in frequency to the pitch of a toneproduced by a musical instrument. For use with electrified instruments,such as electric guitars, which come equipped with an electrical pick-upfor converting the sound made by the instrument into electrical signals,such feeding means is normally wiring connecting the pick-up to thefilter. For use with non-electrified instruments, such feeding meansnormally includes an appropriate pick-up as well as the connectingwiring.

For use in tuning musical instruments, the filter is set to transmit asignal of a frequency corresponding to the desired pitch to be obtainedfrom the instrument. The instrument is then played, and, if thefrequency of the signal it produces corresponds to the set pitch, thesignal is transmitted to the indicating means. If the signal is nottransmitted, tuning of the musical instrument proceeds until theindicating means shows that transmission of the generated electricalsignal has occurred.

THE DRAWINGS The presently contemplated best mode of carrying out theinvention is illustrated in the accompanying drawings in which:

FIG. 1 is an exterior perspective view of the device as constructed foruse with electric guitars, both a guitar and its amplifier beingindicated; and

FIG. 2 is a schematic of the electronic circuitry of the device of FIG.1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The illustratedembodiment of the invention is specifically designed for use in thetuning of guitars, but it will be apparent that other embodiments of theinvention can be constructed from the teachings hereof by those skilledin the electronic art for use in the tuning of any musical instrument.

Referring to FIG. 1, in its illustrated form the device comprises a box10 having an input jack 11 adapted to receive a plug 12, which may beconnected by standard wiring l3 and a plug 14 to a pick-up 15 of anelectric guitar 16. Pick-up 15 converts the musical tones produced byguitar 16 into electrical signals, whose frequency is the same as thatof the pitch of the tone produced. On an electrified instrument, such asthe electric guitar 16, the pick-up referred to may be the normal onesupplied with the instrument, as indicated. On a non-electrified guitaror musical instrument of any kind, the pick-up may be of any standardtype or an ordinary microphone.

Within box 10 is located the circuitry of FIG. 2, which includes anadjustable, active, band-pass filter, made up of operational amplifier17, capacitors C 1, C2, and C3, fixed resistors Rla to Rlf, R2a to R2f,and R3a to R3f, and variable resistors Rlg to R11, R2g to R21, R3g toR31, and R4a to R4f. Such band-pass filter is adjustable by means of athree-pole rotary switch S1, FIG. 1, having the poles Sla, Slb, and 810,FIG. 2, controlling introduction of sets of fixed and variable resistorsinto the circuit. Power is supplied by batteries B1 to B4 throughresistor R5, Zener diodes D1 and D2, capacitors C4 and C5, and pole 52bof a DPDT (double pole, double throw) of a rocker switch S2.

The circuitry also includes indicating means made up of meter 18, diodesD3 to D6, and resistor R6.

An output plug 19 from box 10 is adapted for plugging into input jack 20of the usual amplifier 21 used with an electric guitar. This is aconvenient arrangement, since the tuning device can be plugged in andleft in place during normal playing of the guitar. Switch S2 providesfor checking tune of the guitar at any time the player wishes to do so.

In operation, electrical signals from pick-up 15 enter the band-passfilter through capacitor C6 and resistor R7. Capacitor C6 prevents anyDC value that might be present from entering the filter, and resistor R7attenuates the signal to prevent overload or saturation of the filterand helps determine the gain of the amplifier. It is preferred that thetime constant of the combination R7 and C6 be greater-than the timeconstant of the filter circuit and that R7 should be less than IOOKohms.

The band-pass filter utilized by this invention is an active filter, ascontrasted with a passive filter. As shown in FIG. 2, it is of the typeknown as a twin-tee filter; however, any type of filter network that canbe adjusted to pass essentially a single frequency can be used.

In the twin-tee filter shown, the operational amplifier 17 is preferablya type 709 or 741, such as a Fairchild u5B7709393 or u5B774l393. Type741-is most preferred, because of its higher voltage gain and the factthat it does not need external frequency compensation.

Frequency control of the filter is achieved by reason of the variousresistive and capacitive elements ar ranged in a twin-tee configurationand located between the output and input of the amplifier so as toprovide feedback to the input of the amplifier. The twin-tee networkincludes the capacitors C1, C2, and C3, the fixed resistors Rla throughRlf, R2a through R2f, and R3a through R3f, the variable resistors Rlgthrough R11, R2g through R21, and R33 through R31, and the three polerotary switch S1.

Capacitors C1, C2, and C3, are always connected in the circuit and areselected so that the impedance of C1 equals that of C2 within 10.5percent and that of C3 equals twice that of C1 within $0.5 percent. Thecapacitance values are much larger than any possible stray capacitancesin the circuit, so that such stray capacitances are effectively absorbedby the larger capacitors and donot affect the frequency characteristicsof the circuit. The preferred values of C1 and C2 are 0.1 uF and the,preferred value of C3 is 0.2uF.

As'previously indicated, sets of fixed and variable resistorsare.selectively placed in the filter circuit by means .of switch S1. Thereare six positions of the switch, a position for each string of guitar16. In each position of the switch, three sets of the resistors areelectrically placed in the circuit, each pole of the switch connectingone set into the circuit. The variable resistor of each set is used insetting the exact frequency of the filter to correspond to any givenin-tune musical ,note.

In FIG. 2, the six positions of the switch are designated E, A, D, G, B,and E respectively, each letter representing the in-tune note of one ofthe six open strings of guitar 16. Poles Sla, Slb, and 810 are shown inthe first position E. In such position the set of fixed resistor Rla andvariable resistor Rlg is placed in the circuit by switch pole Sla; theset R and R2g by pole Slb; and the set R3a and R3g by pole Slc. Theamount of resistance placed in the circuit determines the trapped, i.e.,resonant, frequency of the twin-tee network of the filter. A differentvalue of resistance is required for tuning each sound-producing elementof a musical instrument.

Poles Sla and Slb also selectively place feedback variable resistors R4athrough R4f in the filter circuit. One of these resistors is placed inthe circuit in each position of switch S1. With S1 in position shown,R4a is in the circuit. The purpose of the feedback resistors is toalways provide enough negative feedback to the operational amplifier toprevent it from becoming unsta- ,ble and oscillating. The particularvalues of resistance used depend upon the particular type of operationalamplifier used.

The twin-tee arrangement of capacitors and resistors provides a negativefeedback signal to the input of the operational amplifier. At theresonant frequency of the twin-tee, as varied by the resistance in thecircuity, a minimum amount of negative feedback appears at the input tothe amplifier, thereby making the gain through the amplifier maximum.The gain (output voltage divided by input voltage) of the circuit ofFIG. 2 is approximately constant over its six selectable resonantfrequencies. Using a 709 type operational amplifier, the gain is about3700 or 7ldB. The Q of a filter circuit is defined as the resonantfrequency of the filter divided by the bandwidth of the pass curve at apoint 3db down the curve. For example, in the circuit of FIG. 2, using a709 type operational amplifier, with the switch S1 set in the D positioncorresponding to a frequency of 146.8

4 Hz. the bandwidth at 3db down the curve is 0.38 cycles, making the Qequal to 386. The Q of the circuit will vary in each position of switchS1, but with the circuit shown, stays above 100. If a 741 operationalamplifier is used, the gain is increased and the Q increased.

The value of Q is not critical. However, a higher Q (meaning a sharperpass curve) willresult in a more accurately tuned pitch.

In the illustrated embodiment, resonant frequencies of the twin-teearrangement are designed to correspond to the open string frequencies ofthe six guitar strings. These frequencies are:

82.4 Hz. 110.0 Hz. 146.8 Hz. 195.9 Hz.. 246.9 Hz. 329.6 Hz.

To achieve the above frequencies, the values of the resistors in thefilter circuit of FIG. 2 are as follows:

The variable resistors are adjusted until the exact resonant frequenciesare obtained. This is normally done in the manufacture of the device.The person using the tuning device does not adjust the resistors. Theuse here of variable resistors is merely a practical way to achieveexact circuit resistances in a production environment.

The output of operational amplifier 17 is connected to resistor R6 ofthe indicating means, which in turn, is connected to the full waverectifying bridge comprising diodes D3 through D6. Meter 18 is connectedacross the bridge. The signals from the band-pass filter are rectifiedby the diode bridge and the meter is deflected by the rectified current.It is preferred that resistance value of R6 be selectedso that themaximum output of the filter will cause a meter deflection of aboutpercent. However, R6 could be selected to give any desired maximumdeflection.

Many types of indicating means can be used. For example, a lamp whichgoes on when a certain voltage is applied across it could besubstitutedfor the meter in the bridge circuit and R6 adjusted. so thatmaximum output of the filter would produce the necessary voltage. Again,the entire indicating means shown in FIG. 2 could be replaced by aloudspeaker. The output of the filter would then cause the loudspeakerto produce a tone which would reach maximum volume with maximum filteroutput. It should be realized that many other indicating means could beused.

With switch S2 in the position shown, pole 82b is closed so as toconnect batteries B1, B2, B3, and B4 with the rest of the power supplycircuit. These batteries are each nine volts and of the type commonlyused in transistor radios.

The batteries are connected in series to produce a total of 36 volts.Zener diodes D1 and D2 are rated at 16 volts and biased by RS, so thatthey carry only about one quarter as much current as the operationalamplifier. R5 drops about 4 volts, leaving 16 volts across each Zenerdiode. As the batteries weaken with use and age, the Zener diodescontinue to drop a constant 16 volts each. However, the voltage droppedby RS decreases. The ground connection between diodes D1 and D2 enablesthe power supply to provide +16 volts and l6 volts with respect toground. The capacitors C4 and C5 prevent any undesirable AC signals thatmight feedback through the power supply from being transmitted alongwith the DC to the operational amplifier.

Pole 52a of switch S2, in the position shown, connects jack 11 to thecircuitry of the band-pass filter. If switch S2 is placed in itsalternate position, jack 11 is directly connected electrically to theoutput plug 19 of box 10 by pole S2a. Pole 82b in this position opensthe power supply circuit, so that no power is supplied to amplifier 17.Switch S2 acts as an on-of switch, and also, when it is off position,causes the signals from the instrument to by-pass the tuning circuitryand to proceed directly from jack 11 to plug 19.

In using the illustrated device as interconnected between guitar 16 andits amplifier 21, switch S2 is pushed into its tune position wheneverthe player wishes to test the tune of the guitar. Switch S1 is rotatedto the position corresponding to the guitar string (E, A, D, G, B, or E)desired to be tuned. The string is plucked and the response of meter 18watched. If out of tune, tension of the string is adjusted in the usualmanner until the meter reaches a maximum deflection, indicating that thestring is in tune. Switch S1 is then rotated to the positioncorresponding to the next string to be tuned, and that string isadjusted until the meter indicates that it is in tune. This process isrepeated until all strings have been tuned. Thereupon, switch S2 isswitched to play position, and normal play is resumed. Anytime it isdesired to retune the instrument, S2 is again switched to tune, theguitar tuned if required and switch S2 then returned to play position.

Although this detailed description is specific to guitar tuning, it isobvious that the device can be adapted for use with any musicalinstrument. Switch S1 or a plurality of similar switches would provide aposition for each sound-producing part of the instrument. Such switch orswitches would position various resistances in the filter circuit sothat the pass frequency of the filter would correspond to the in-tunenote produced by the particular part of the instrument concerned. Thevalues of the resistances necessary can be found by ordinarycalculations performed by anyone skilled in the electronics art. If alarge range of notes is to be covered, such as in the tuning of a piano,the capacitance values of the filter circuit may also be changedselectively in similar manner.

Whereas this invention is here illustrated and described in detail withrespect to a preferred specific embodiment thereof, it should beunderstood that various changes may be made without departing from theinventive concepts as defined herein and particularly pointed out in theclaims.

What I claim is:

1. A device for testing the tune of musical instruments comprising aprecalibratcd, stepwise adjustable, active, band-pass filter fortransmitting at any given time an electrical signal of substantiallypredetermined frequency blocking other signals; indicating meansarranged to be operated in response to signals transmitted by saidband-pass filter; means for feeding to the band-pass filter electricalsignals corresponding in frequency to the pitch of a tone produced by amusical instrument; by-pass circuitry interconnectable with the feedingmeans; means for connecting said by-pass circuitry with an outputamplifier; and switching means for connecting either said band-passfilter or said bypass circuitry with said feeding means; and means forproviding power to the band-pass filter.

2. A device in accordance with claim 1, wherein the band-pass filtercomprises circuitry of twin-tee type.

3. A device in accordance with claim 2 wherein the twin-tee circuitrycomprises fixed capacitors, sets of fixed and variable resistances, andat least one selection switch for operatively connecting certain of saidsets at any given time.

4. A device in accordance with claim 3, wherein the values of the fixedcapacitors are symmetrical.

5. A device in accordance with claim 1, wherein the means for connectingsaid by-pass circuitry with an output amplifier is a standard male plug,and wherein the device is mounted in a housing rigidly connected to saidplug whereby the device is held in place when said plug is connected tothe output amplifier.

1. A device for testing the tune of musical instruments comprising aprecalibrated, stepwise adjustable, active, bandpass filter fortransmitting at any given time an electrical signal of substantiallypredetermined frequency blocking other signals; indicating meansarranged to be operated in response to signals transmitted by saidband-pass filter; means for feeding to the band-pass filter electricalsignals corresponding in frequency to the pitch of a tone produced by amusical instrument; by-pass circuitry interconnectable with the feedingmeans; means for connecting said by-pass circuitry with an outputamplifier; and switching means for connecting either said bandpassfilter or said by-pass circuitry with said feeding means; and means forproviding power to the band-pass filter.
 2. A device in accordance withclaim 1, wherein the band-pass filter comprises circuitry of twin-teetype.
 3. A device in accordance with claim 2 wherein the twin-teecircuitry comprises fixed capacitors, sets of fixed and variableresistances, and at least one selection switch for operativelyconnecting certain of said sets at any given time.
 4. A device inaccordance with claim 3, wherein the values of the fixed capacitors aresymmetrical.
 5. A device in accordance with claim 1, wherein the meansfor connecting said by-pass circuitry with an output amplifier is astandard male plug, and wherein the device is mounted in a housingrigidly connected to said plug whereby the device is held in place whensaid plug is connected to the output amplifier.